If there is a failure to write to the SD card, what makes you think that a reboot of the Arduino will fix the problem?

After some hours of operation, a few arduinos get hanged (not all every time, I have 5). This I figure out looking at the SD led.They again start working when I press the reset button and led turns on.

The WDT with a long timeout should be fine (eg. 8 seconds). Just "pat the dog" in the main loop, when things are going fine and the watchdog will reset it after 8 seconds if not.

Thanks Nick.I have 30 sec of delay in my main loop. Will it affect soft resetting?Probably I don't understand this 8 sec of wait, I am having a doubt that this will reset while Arduino is in waiting loop of 30 sec.Please clarify.

If you just want to use the WDT for reset, you should disable it completely in setup (or, ideally, the bootloader) and only enable it when you want to reset. Unless, of course, you want an actual watchdog for your rc car or transformer or whatever.